I have a S50 and a lexar 512MB x12 speed card.
I compared the write speed in a very crude "how does it feel like" test to the 32MB card supplied with the camera.

In 5mp with superfine resolution I hardly noticed any difference.
In 5mp raw mode, the lexar was a little fast than the supplied card.
The first images were bufferd, but it took a little longer to get the 3rd picture after the second one was shot and was being written.

I had no problems taking videos with either card at any resolution.

As far as USB2, if you get a USB2 reader and an ultra fast card then you *might* encounter a difference in read speed.

For my type of use its a non-issue, I just start downloading the pictures and get myself a coffee.

I doubt it that for regular use, one can find a real difference in operating this camera with cards faster than 12x-16x
.
If I am wrong about it, I would be very interested to hear otherwise.